Field of the Invention
This invention relates generally to aircraft transparencies and, more particularly, to an aircraft transparency having a pressure seal incorporating integrated compression stops and/or having an anti-static drain assembly.
Technical Considerations
Commercial passenger aircraft have a pressure-tight fuselage to allow the interior of the aircraft to be pressurized during flight. In order to maintain the pressure integrity in the interior of the aircraft, as well as to prevent moisture penetration, aircraft transparencies (for example cockpit windshields and cabin windows) must remain tightly sealed against the aircraft body. However, during normal flight operations in which the aircraft travels from ground level to thousands of feet in the air and then back again, the aircraft fuselage expands and contracts due to the pressure differential between the interior of the aircraft and the external environment. In order to maintain a pressure-tight seal between the aircraft transparency and the aircraft fuselage during expansion and contraction of the fuselage, a flexible gasket or “pressure seal” is positioned between the transparency and the aircraft fuselage. When the transparency is attached to the fuselage, typically by bolts, the pressure seal is compressed between the outer edge of the transparency and the fuselage as torque is applied to the bolts. The flexible pressure seal helps to maintain a pressure-tight and moisture-proof seal between the fuselage and the transparency even as the fuselage expands and contracts during normal operations.
The torque applied to the bolts when attaching the aircraft transparency to the fuselage is very important. If the torque is too high, the pressure seal can become overly compressed and lose its range of expansion and contraction. If the torque is not high enough, the aircraft transparency may not be tightly sealed against the aircraft fuselage. Even if the correct amount of torque is applied, other factors may adversely impact upon the operation of the pressure seal. For example, the material of the pressure seal may lose flexibility and retain its compressed state (in flight state) after extended periods of time and temperature cycling. This can create a gap between the mating surfaces of the transparency and the fuselage, which can allow moisture penetration into the aircraft fuselage as well as a loss of pressure. Further, as the gasket material loses its flexibility over time, the bolts securing the transparency to the fuselage may deflect. This reduces the torque on the bolts and allows the bolts to become loose (conventionally referred to as “creep”).
Another problem associated with conventional aircraft transparencies is precipitation static (known as “P-static”). Precipitation static is an electrical charge that builds up on the exterior of the aircraft transparency when the aircraft flies through certain environmental conditions, for example ice particles, rain, snow, and dust. Precipitation static builds up on the outside surface of the aircraft transparency until the static charge reaches a critical point, at which point it suddenly discharges to the adjacent metal aircraft fuselage. This sudden electrical discharge can disrupt aircraft communications, as well as navigation and surveillance radars, and can damage the aircraft radomes and transparencies. Precipitation static can also lead to “streamer noise” caused by charge buildup on nonconductive aircraft areas, such as windshields. This charge buildup and subsequent discharge can disrupt aircraft communications, particularly high UHF frequencies, and has the potential to cause interference with antennas close to the discharge source. Also, under certain conditions, a sudden static electrical discharge may damage the transparency, causing failure of the transparency heating system and/or partial loss of visibility through the transparency.
It would be advantageous to reduce or eliminate at least some of these problems associated with conventional aircraft transparencies. For example, it would be advantageous to provide a pressure seal for an aircraft transparency that reduces the likelihood of creep and/or torque loss during aircraft operations. For example, it would be advantageous to provide a method and/or structure to allow the precipitation static on the aircraft transparency to be drained away from the transparency before sudden discharge to reduce the disruption of aircraft communications and/or navigation and/or damage to the aircraft transparency.